1. Field of the Invention
The present invention relates to a wavelength multiplex transmission device in the field of optical communication and, more particularly, to a wavelength multiplex transmission device enabling in-service expansion of signal lights without cutting off existing signal lights.
2. Description of the Related Art
In the recent field of optical communication, the mainstream is wavelength multiplex transmission which intends to expand a transmission capacity by multiplexing different signal wavelengths, in which a system equivalent to 10 Gb/s and 64 waves is operated. Even with a maximum transmission capacity of 64 channels, however, it is a common practice to have a less number of signal wavelengths introduced at initial operation than that and sequentially add signal lights according to a future demand. In the course of the expansion, neither error in signal light during service nor cut-off of a cable should be generated.
FIG. 7 is a block diagram showing a conventional wavelength multiplex transmission device. In the following, the device will be described with reference to the drawings. Although the total number of signal wavelengths is 48 here, basic structure of the device is the same regardless of the number of wavelengths.
In submarine cable communication, because a transmission distance is extremely long, dispersion in a transmission path causes deterioration of a signal waveform. “Dispersion” here represents a delay of a signal pulse due to a difference in a group velocity with respect to a wavelength. As a result, the longer a transmission distance becomes, the greater the effect of a delay on waveform deterioration becomes. Therefore, by applying reversal of the amount of dispersion generated on a submarine transmission path to a transmission device to compensate for the dispersion, waveform deterioration is suppressed.
In such dispersion compensation as described above, it is only necessary to apply a predetermined amount of dispersion individually to each signal. This method, however, is inefficient because device arrangement would be extremely large. Therefore, as illustrated in FIG. 7, it is efficient to divide the entire signal band into three, an S band, an M band and an L band and apply common dispersion to each band to compensate for shortage or excess of each signal on an individual channel basis. Since structure of the S band, the M band and the L band is the same, description will be made in the following with respect to the S band. Subscript n attached to a reference numeral denotes any of the integers from 1 to 16.
Signal lights DATA 1 to DATA 16 are applied to a dispersion compensating fiber unit 1S to have their shortages/excesses compensated for, propagated through an optical switch unit 10S and amplified by a channel optical amplifier unit 2S, and then all the signal lights of the S band are multiplexed at a wavelength multiplexing unit 4S. While an optical switch 10S-n initially transmits a CW (continuous wave) light source 11S-n side, when a signal light is introduced, it is switched to transmit the signal light. In other words, the optical switch 10S-n corresponding to signal light power yet to be added receives input of the CW light.
The signal light having transmitted through the wavelength multiplexing unit 4S is uniformly subjected to block dispersion compensation by a dispersion compensating fiber 5S-1 of a block dispersion compensating unit 5S, is transmitted through a band optical amplifier 5S-2 for compensating for a pass loss at the dispersion compensating fiber 5S-1 and is then applied to a band multiplexing unit 8.
The foregoing is the description made of the S band, which is also the case with the M band and the L band. Then, the signal light and the CW light are output from the band multiplexing unit 8 and after being amplified to predetermined power by a total band optical amplifier 9, output to a transmission path.
In common band amplifier and total band amplifier, wavelength dependency of a gain changes with input power. Therefore, if no CW light source exists, signal light power output onto the transmission path will vary in an initial mode and a final mode where expansion of the total signal lights is completed, so that an error might occur due to a shortage of a gain in a signal light of a certain wavelength. To prevent such problem, supplement with a CW light is made in advance.
The above-described conventional wavelength multiplex transmission device, however, has the following shortcomings.
The device is structured such that a CW light for supplementing signal light power in preparation for future expansion propagates through the dispersion compensating fiber 5S-1 and the band optical amplifier 5S-2 for compensating for a pass loss, etc. of the block dispersion compensating unit 5S.
Since the block dispersion compensating unit 5S is an optical part necessary for suppressing waveform deterioration of a signal light, the CW light needs not to be propagated to the block dispersion compensating unit 5S. In other words, the structure of the conventional wavelength multiplex transmission device in which a CW light propagates through the block dispersion compensating unit and the like at the time of initial operation is adopted although it is not an essential structure for satisfying transmission characteristics, which is a factor of an increase in initial costs of the device and device scale as well.